To obtain chemical energy from solar energy, i.e., to utilize hydrogen energy, which is infinite and clean, is a dream of human beings. Energy issues in the 21st century and the global warming effect of carbon dioxide produced by fossil energy as well as environmental pollution including acid rain can be resolved by putting that energy into practical use.
The Honda-Fujishima effect published in A. Fujishima et. al., Nature, 238, 37(1972) was the first attempt which showed that water can be decomposed into oxygen and hydrogen with light energy. Thereafter, in the days when the oil crisis aroused questions all over the world, many investigations were actively made on techniques for converting light energy into chemical energy based on that principle. However, no improvement in the efficiency of light energy conversion in the visible light region has been made so far. A result of the active investigations made in the days of from 1980 to 1990 is that it was demonstrated that the electrons and holes generated by optical excitation recombine before reaching reaction sites for water decomposition and this recombination governs the efficiency of conversion. It was attempted to utilize an intercalation compound in order to separate reaction sites in view of that conclusion (S. Ikeda et. al., J. Mater. Res., 13, 852(1998)). Although the efficiency of conversion has been gradually improved, a satisfactory conversion efficiency in the visible light region has not yet been attained. This is because complete separation of reaction sites, i.e., separation between electrons and holes, has not been attained.
Simultaneously with the investigations described above, investigations were made on a reaction system for yielding hydrogen by utilizing light absorption by ions in a solution. It was reported in J. Jortner, et. al., J. Phys. Chem., 68, 247 (1964) and K. Hara, et. al., J. Photochem. PhotoBiolo. A128, 27 (1999) that hydrogen is yielded at a high quantum efficiency in an acidic solution containing iodine ions and in an alkali solution containing sulfur ions, respectively. However, all these reactions are possible with high-energy ultraviolet light having a wavelength of 250 nm or shorter.
On the other hand, since photocatalysts have the property of accelerating various chemical reactions such as, e.g., the decomposition of environmental pollutants, malodorous ingredients/various bacteria, or the like, they have come to be practically used in applications such as tiles having antibacterial activity and antibacterial/deodorizing filters for air cleaners. Furthermore, it is possible to cause a photocatalyst to act on a harmful substance to obtain a useful chemical substance therefrom. For example, photocatalysts are expected to be applied to a crude oil desulfurization step.
In the step of desulfurizing crude oil which is generally conducted presently, heavy naphtha is subjected to hydrofining during crude oil distillation to recover all the sulfur ingredients contained in the crude oil as hydrogen sulfide. This hydrogen sulfide is recovered after oxidation of sulfur through the process called Claus process. The Claus process is a process in which one-third of the hydrogen sulfide is oxidized into sulfurous acid gas and this gas is reacted with the remaining hydrogen sulfide to obtain elemental sulfur.
This process necessitates an enormous amount of energy because of not only the catalytic reaction of sulfurous acid gas with hydrogen sulfide but also repetitions of heating and condensation. It further has problems, for example, that the management of sulfurous acid gas is costly.
If a method which comprises adding a photocatalyst to an aqueous alkali solution containing hydrogen sulfide dissolved therein, irradiating the resultant mixture with light to thereby cause the photocatalyst to absorb the light energy of the radiation and generate free electrons and free holes, and oxidizing/reducing the aqueous alkali solution containing dissolved hydrogen sulfide by the free electrons and holes to obtain hydrogen and sulfur, i.e., a method for decomposing hydrogen sulfide with a photocatalyst to yield hydrogen and sulfur, can be put to practical use, it becomes possible to decompose hydrogen sulfide as a harmful substance with a smaller amount of energy to produce hydrogen and sulfur as useful substances. Namely, this technique contributes to the resolution of environmental problems and enables the useful substances to be produced at low cost.
However, the photocatalysts proposed so far have had the following problems which should be overcome. First, the catalytic activity is low. Secondary, the photocatalysts are toxic. Although the photocatalysts generate free electrons and free holes upon irradiation with light, the probability that the free electrons recombine with the free holes is high. Furthermore, the probability that chemical substances formed through decomposition by oxidation/reduction reactions recombine with each other and return to the original compound is also high. Because of these, the catalytic activity is low.
Thirdly, the catalysts have a short life. The prior-art photocatalysts have the following problem concerning photodissolution. Although the photocatalysts generate free electrons and free holes upon irradiation with light, the catalysts themselves are oxidized/reduced besides the target chemical substance because of the high susceptibility to oxidation/reduction reactions of these electrons and holes. The catalysts thus dissolve away and are deprived of their catalytic activity.
For overcoming these problems, JP-A-2001-190964 disclosed a photocatalyst having high catalytic activity, no toxicity, and a long life. Thus, the three problems described above were eliminated.
However, the photocatalyst disclosed in JP-A-2001-190964 is limited to one comprising ZnS. Since the band gap for ZnS is in the ultraviolet region, it has been impossible to utilize visible light such as, e.g., sunlight, which is infinite clean energy, as it is for photocatalytic reactions.